Fix traitlet float64 warning

[bmorris3]

In Frequency Analysis, we plot the periodogram like so:

lcviz/lcviz/plugins/frequency_analysis/frequency_analysis.py

Line 172 in b4bf32a

line.x, line.y = getattr(per, self.xunit_selected), per.power

Since the periodogram returns Quantitys for period and power, the traitlet can't successfully validate the array input, raising:

UserWarning: Given trait value dtype "float64" does not match required type "float64". A coerced copy has been created.

This PR extracts the Quantitys' values as ndarrays to avoid the warning.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
lcviz/events.py	100.00% <100.00%> (ø)
lcviz/helper.py	100.00% <ø> (+2.85%)	⬆️
lcviz/plugins/coords_info/coords_info.py	95.04% <100.00%> (ø)
lcviz/plugins/ephemeris/ephemeris.py	94.57% <100.00%> (ø)
...z/plugins/frequency_analysis/frequency_analysis.py	91.34% <100.00%> (ø)
lcviz/template_mixin.py	84.44% <100.00%> (-1.84%)	⬇️
lcviz/utils.py	92.03% <100.00%> (+0.29%)	⬆️

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